In the flexible packaging industry the use of thermoplastic sheet materials to package or enclose various products often requires the sealing together of the sheet materials by adhesives or heat to make a complete receptacle. However, seal integrity becomes difficult to achieve and maintain when the package is subjected to abusive handling and to media of differing temperatures, particularly high temperatures that approach or exceed the boiling temperature of water. This is especially true in certain thermoplastic materials which seal readily but form seals weaker than are desired. Such seals often occur when polyethylene or ethylene-vinyl acetate copolymer materials (hereinafter called "EVA") are joined.
Single layer, self-supporting films may be made from polyethylene or from EVA or a layer or layers of a laminate may be made from polyethylene or EVA; and, sometimes, it is desirable that the polyethylene or EVA layer of a laminate be crosslinked chemically or irradiatively to improve the strength properties thereof. However, no matter how strong a packaging film is, obviously, a package made from film is no stronger than the seals made in the film. Thus, it is one object of the subject invention to provide a method of strengthening the seals of crosslinkable packaging materials, particularly, strengthening the high temperature seals.
To produce a seal, certain prior art patents have suggested that prior to any sealing that the area to be sealed be irradiated so that the effects of radiation alone produce the seal. For example, in U.S. Pat. No. 2,997,419 which issued on Aug. 22, 1961 to E. J. Lawton, two sheets of polyester material are welded together by irradiating the contacting sheets with high energy electrons but the energy from the electrons alone is responsible for the welding of the polyester. Accordingly, it is another object of the present invention to enhance the strength of a seal previously formed by heat between thermoplastic materials. In another prior art patent, U.S. Pat. No. 2,936,261 which issued on May 10, 1960 to Q. P. Cole, a method of sealing together irradiated polyethylene sheets is described wherein the sealing is achieved by interposing a polymeric material between the layers which has a curing or cross-linking agent therein. However, it is an object of the present invention to improve the strength of heat seals between thermoplastic materials without the use of intermediate compounds or materials. One such improvement is shown in U.S. Pat. No. 4,127,688 which issued to Anne C. Bieler et al. on Nov. 28, 1978.
A necessary feature for packaging many food items is that the packaging material be relatively impervious to gases such as nitrogen, oxygen, and carbon dioxide. A proven flexible plastic material for this purpose is the copolymer of vinylidene chlorie commonly called "saran." However, a mono-layer saran film does not possess the abuse resistance required for present food distribution systems and to overcome this deficiency multi-layer films are used with saran as an interior layer. While such multi-layer films have found successful uses in low and moderate temperature ranges their use in high temperature applications has been limited as the exterior, heat sealing layers are usually polyolefins such as polyethylene and ethylene-vinul acetate with the deficiences mentioned above. Saran/polyolefin laminates have been successfully cross-linked by irradiation as taught by U.S. Pat. No. 4,044,187 which issued Aug. 23, 1977 to William P. Kremkau but one disappointing result of irradiating saran is that it tends to degrade and turn an unsightly brown color. Thus, an object of this invention is to provide an alternative to an irradiated saran/polyolefin laminate.
These and other objects are achieved by the present invention which is described below.